The present invention generally relates to electrophotography, and more particularly, to a color electrophotographic process and an apparatus employed therefor which may be applied to a printer, facsimile device or the like.
In the field of the color electrophotographic apparatus as referred to above, there has conventionally been proposed, for example, an apparatus in which four image forming units each containing a photosensitive member or photoreceptor and a color developing material are successively arranged along a paper transport path so that development in full color may be effected according to colors, as disclosed, for example, in Japanese Patent Laid-Open Publication Tokkaihei No. 2-16580 as shown in FIG. 1.
In FIG. 1, there is illustrated only a printer portion, i.e. an image forming portion of a copying apparatus disclosed in said Tokkaihei No. 2-16580. Digital signals sent from an image reading portion (not particularly shown) of the apparatus, are inputted to the printer portion, and fed to laser light exposure devices of color signals respectively for black 1BK, cyan 1C, magenta 1M, and Yellow 1Y at which four recording units 2BK, 2C, 2M, and 2Y are arranged side by side as shown. Since each of said recording units is composed of the same constitutional parts, description will be given only with respect to the recording unit 2C for cyan, and explanation for those of other colors is abbreviated for brevity, with like parts being designated by like reference numerals affixed with letters representing colors for quick reference. Here, the representative recording unit 2C is provided with a photosensitive or photoreceptor drum 3C besides the laser exposure device 1C. Around the photoreceptor drum 3C, there are sequentially disposed various processing stations such as a corona charger 4C, the laser exposure device 1C, a developing unit 5C and a transfer charger 6C, etc. The photosensitive surface of the photoreceptor drum 2C uniformly charged by the corona charger 4C is formed thereon with a latent image of cyan light image through exposure by the exposure device 1C and the latent image is developed in a visible image by the developing unit 5C.
Thus, copy paper (not shown) is successively fed by a transfer belt 7 in the form of an endless belt movably supported by a pair of rollers between the photoreceptor drums 3BK, 3C, 3M, and 3Y respectively formed with visible images in the above described manner, and is transferred with the visible images by the action of the transfer units 6, whereby a full color image may be obtained on the copy paper.
Moreover, in the color image electrophotographic apparatus generally used, a practice to form the color image by overlapping toner images in respective colors for yellow, magenta, cyan and black has been adopted, and a general process for effecting the overlapping of such toner images on the transfer material is described, for example, in a color image forming apparatus disclosed in Japanese Patent Laid-Open Publication Tokkaihei No. 1-252982.
FIG. 2 schematically shows an overall construction of the prior art color image forming apparatus referred to above.
In FIG. 2, the known color image forming apparatus generally includes a photosensitive or photoreceptor drum 8 rotatably provided approximately at a central portion of an apparatus housing H, and various processing stations such as a corona charger 9, a developing section 10, a transfer drum 11, and a cleaner 12, etc. sequentially disposed around the photoreceptor drum 8. The developing section 10 constituted by a Y developing unit 13 for forming toner image in yellow, an M developing unit 14 for toner image in magenta, a C developing unit 15 for toner image in cyan, and a B developing unit 16 for toner image in black is arranged to be rotated on the whole so that respective developing units successively confront the photosensitive surface of the photoreceptor drum 8 so as to be in the state capable of effecting the developing. The transfer drum 11 and the photoreceptor drum 8 are respectively rotated in directions indicated by arrows as they confront each other during operation.
In the first place, upon starting operation, the photoreceptor drum 8 is rotated in the direction indicated by the arrow, with the photosensitive surface thereof being uniformly charged ty the corona charger 9.
Thereafter, the photosensitive surface of the photoreceptor drum 8 is irradiated by the laser beam modulated through the signal for forming the image of the first color yellow, whereby the latent image is formed thereon, which is further developed by the developing unit 13 for yellow firstly confronting said photoreceptor drum 8, so as to be formed into a yellow toner image. Before the yellow toner image thus formed arrives at the position confronting the transfer drum 11, a copy paper sheet fed from a paper feeding section 18 has already been wound onto the outer peripheral surface of said transfer drum 11, with the leading edge of the copy paper sheet being grasped by a claw portion 19, and the toner image is formed at a timing in which the yellow toner image on the photoreceptor drum 8 faces and meets predetermined position of the paper sheet.
After the yellow toner image on the photoreceptor drum 8 has been transferred onto the paper sheet by the action of a transfer charger 20, the photosensitive surface of the photoreceptor drum is cleaned by the cleaner 12 so as to be prepared for the formation of the image of the next color. Subsequently, toner images for magenta, cyan, and black are formed in the similar manner, at which time, the developing section 10 causes each of the developing units to be used according to the colors to confront the photoreceptor drum 8 so as to be ready for the developing. The transfer drum 11 has a sufficient diameter so that the change-over of the developing units is effected in time and the toner image of the previous color is overlapped with that of the next color.
The irradiation of the laser beam L for the image formation of each color is effected in such a timed relation that, as the photoreceptor drum 8 and the transfer drum 11 rotate, the toner image of each color on the photoreceptor drum 8 and the toner image already transferred onto the paper sheet over the transfer drum 11 are positionally aligned or registered with each other. In such a manner, the toner images in four colors are transferred onto the paper sheet in the superposed state over the transfer drum 11, thereby to form the color image on said paper sheet. After the toner images for all colors have been transferred thereon, the paper sheet is separated from the transfer drum 11 by a separating claw 21, and then, fixed with the toner image by a fixing unit 23 through a transfer section 22 so as to be subsequently discharged out of the apparatus. Given so far is the brief description of the construction and function of the conventional color image forming apparatus.
However, in the arrangement as described with reference to FIG. 1, there has been such a problem that the construction of the developing apparatus tends to be on a large scale, and the recording unit for each color becomes large and complicated, while four laser exposure devices are required, thus resulting in high cost.
Meanwhile, in the arrangement of FIG. 2 in which the transfer drum 11 is employed for aligning the positions of the toner images in different colors as overlapped, it is necessary to once winding the paper sheet around the transfer drum for transferring the toner image on the photoreceptor drum onto said paper sheet, even when an image in a single color is to be formed through employment of only one of the developing units, and thus, copying speed for the single color mode becomes slow as compared with that of an usual single color copying apparatus.
Moreover, since the transfer drum originally unnecessary during the single color copying is operated, maintenance and replacement of parts particular to the transfer drum are also required in the similar manner as in the color copy, even when a large amount of single color copying is effected in the above color image forming apparatus of FIG. 2.
Furthermore, in the arrangements of FIGS. 1 and 2, when color images are continuously printed under high temperature circumstances, since the photoreceptor drum is also continuously used, charge potential thereof is gradually lowered due to electrostatic fatigue and temperature rise of the photoreceptor, with a consequent variation in the image quality obtained.